Absorbent structures are useful as absorbent cores in a variety of different products, for example, diapers, incontinent pads, catamenial products, bandages, and the like. Such absorbent structures have generally been comprised primarily of absorbent fibrous materials, such as absorbent papers, absorbent cloths, fibrous batts, and the like.
More recently, a new class of compounds commonly known as superabsorbent polymers have been developed and are gaining increasing use as at least a part of absorbent structures. Superabsorbent polymers as used herein are water-insoluble polymeric materials typically capable of absorbing at least fifteen times their weight of water. Such superabsorbent polymers are available in a variety of forms; for example, they may be a film or a particulate form such as flakes, powders, or granules. Superabsorbent polymers generally differ from many conventional absorbent materials in that once an aqueous fluid is absorbed by most superabsorbent polymers, it generally cannot be expressed from the superabsorbent polymer under moderate pressure. This is often highly desirable in an absorbent structure in that it prevents absorbed fluid from being expressed out of the structure.
When most superabsorbent polymers absorb aqueous fluids, they swell substantially, often to double their dry dimensions or more at saturation. As most superabsorbent polymers absorb fluid and swell, they generally become a gelatinous mass. If the superabsorbent polymer is in a particulate form and the particles are close to one another, they can coalesce and form a gel barrier which can block the flow of fluid.
Absorbent structures which are combinations of absorbent fibrous materials and superabsorbent polymers are well known. Because the superabsorbent polymer is generally present in such a structure in some particulate form, the fibrous material is used to hold the superabsorbent polymer particles within the absorbent structure.
Absorbent structures are known where superabsorbent polymer particles and absorbent fibers are blended to produce an absorbent structure with the superabsorbent particles dispersed throughout the structure. Such absorbent structures are disclosed in U.S. Pat. Nos. 3,661,154 issued to Torr on May 9, 1972, and 3,888,257 issued to Cook & Jackson on June 10, 1975.
More often absorbent structures which combine absorbent fibrous materials and superabsorbent polymers are constructed such that there are one or more layers of each of the two materials. Such layered absorbent structures are disclosed in U.S. Pat. Nos. 3,070,095 issued to Torr on Dec. 25, 1962; 3,371,666 issued to Lewing on Mar. 5, 1968; 3,381,688 issued to Satas on May 7, 1968; 3,654,929 issued to Nilsson, Udden & Wennerblom on Apr. 11, 1972; 3,669,103 issued to Harper, Bashaw & Atkins on June 13, 1972; 3,670,731 issued to Harmon on June 20, 1972; 3,888,256 issued to Studinger on June 10, 1975; 3,901,236 issued to Assarsson, King & Yen on Aug. 26, 1975; 3,959,569 issued to Burkholder on May 25, 1976; 4,055,180 and 4,055,184 both issued to Karami on Oct. 25, 1977; 4,102,340 issued to Mesek & Repke on July 25, 1978; 4,103,062 issued to Aberson & Stulgate on July 25, 1978; 4,105,033 issued to Chatterjee & Morbey on Aug. 8, 1978; 4,232,674 issued to Melican on Nov. 11, 1980; 4,260,443 issued to Lindsay, Meintrup & Slawny on Apr. 7, 1981; 4,269,188 issued to Nishizawa, Shirose & Itoh on May 26, 1981; 4,293,609 issued to Erickson on Oct. 6, 1981; and 4,360,021 issued to Stima on Nov. 23, 1982. Such a layered absorbent structure is also disclosed in U.K. Patent Application No. 2,049,553 published on Dec. 31, 1980.
For absorbent structures where absorbent fibrous materials and superabsorbent polymer particles are present in layers, a number of techniques have been used to secure the superabsorbent polymer particles within the absorbent structure. Torr '095 discloses sifting superabsorbent particles onto a tissue layer and using flat pressure rollers to press the particles into the tissue. Harper, Bashaw & Atkins discloses layering superabsorbent polymer particles between layers of crepe paper and quilting the structure to hold the particles in place. Harmon discloses securing superabsorbent polymer particles in embossed cavities of paper layers and using an adhesive to secure the layers of paper together. Assarsson, King & Yen discloses superabsorbent particles layered between fluff pads which are embossed with a diamond grid pattern. Burkholder discloses superabsorbent polymer particles distributed between layers of tissue, contacting the layered structure with steam to make the particles sticky, and compressing the structure to adhere the particles to the tissue layers. Karami '180 discloses a structure having superabsorbent polymer particles in pockets between a cellulose sheet and a polyethylene film which are adhered around the pockets by adhesive or heat sealing. Aberson & Stulgate discloses a densified bonded layer of fluff pulp and superabsorbent polymer particles which is compressed and heated to fix the particles to the fluff fibers. Melican discloses depositing superabsorbent polymer particles in a predetermined pattern between wetted tissue layers and compressing the resulting structure to adhere the layers together. Lindsay, Meintrup & Slawny discloses layering superabsorbent polymer particles between tissue layers where water is applied at spaced points on the tissue and the layered sheets are embossed to provide points of adhesion such that the particles act as an adhesive between the layers. Stima discloses an absorbent structure having superabsorbent polymer particles in pockets between tissue layers, the tissue layers being adhered by moistening and compressing around the pockets with a grid pattern that matches the pockets. U.K. Patent Application No. 2,049,553 discloses an absorbent structure having superabsorbent polymer particles between two sheets which are adhered by dry embossing between a textured roll and a flat roll such that some areas of the resulting layered structure are highly compressed while other areas are not compressed at all.
It has been found that the absorbent structures disclosed in the above references suffer from one or more of the following disadvantages:
(1) the full absorbent rate and/or capacity of the superabsorbent particles are not capable of being utilized due to physical restraint of the particles or changes in the absorptive rate and/or capacity of the particles caused by the process of making the structure; PA1 (2) fluid is not readily dispersed through the structure or around the particles; consequently, optimum contact between the fluid to be absorbed and the superabsorbent polymer particles is not achieved; PA1 (3) the structure cannot be produced such that it is economically attractive. PA1 (a) n webs of fibrous material, n being an integer of two or more. Each of the webs has two substantially parallel surfaces. The webs are layered such that there is an uppermost web, a lowermost web, n-2 intermediate webs, and n-1 interfaces of two opposed adjacent contacting surfaces of adjacent webs. Each of the interfaces has a surface area. PA1 (b) Absorbent particles which form a discontinuous layer at one or more of the interfaces.